1. Field of the Invention
The present invention relates to springs, such as coils springs, leaf springs, torsion bars, etc., used in, for example, a vehicular suspension system, and more particularly, to a high-strength spring strengthened for lightweight designs or other purposes.
2. Description of the Related Art
Coil springs, for example, can be reduced in weight if their permissible maximum stress or design stress is increased. The design stress of the coil springs depends mainly on their durability and creep resistance. Conventionally, the failure rate of coil springs on the market is extremely low, and their durability arouses no problem. In general, therefore, the improvement of the creep resistance has been the most important problem to be solved.
Heretofore, both the aspects of materials and processing have been taken into consideration in improving the creep resistance of the coil springs. As regards the materials, there is a proposal to use a coil spring steel (SUP7) with increased content of silicon, as an element for increasing the strength of Ferrite, or a coil spring steel (SUP12V) additionally containing vanadium as a crystal grain refining element. In the aspect of processing, on the other hand, the creep resistance used to be improved by warm setting. The SUP7 and SUP12V, which are spring steels defined by Japanese Industrial Standard (JIS) No. G4801, are equivalent to Society of Automotive Engineers (SAE) No. 9260.
When using the prior art as described above, however, the maximum design stress (r max) for the fatigue life not shorter than a fixed level is 110 kgf/mm.sup.2 (1080 MPa), and a higher design stress cannot be obtained. The reason for this will now be described in connection with the conventional spring steels.
The harder the spring itself, the lower the residual shearing strain of the conventional coil spring steels, the SUP7 and SUP12V, is. In other words, if the spring is made harder, then the creep resistance is increased in proportion. The hardness of the spring is expressed in Brinell indentation diameter (hereinafter referred to as HBD). The HBD is the diameter of an indentation formed by pressing a cemental carbide ball, e.g., tungsten carbide ball, 10 mm in diameter into the surface of a sample with a load of 3,000 kgf.
The hardness of the conventional coil spring steels ranges from .phi. 2.70 to 2.90 mm in HBD. In order to obtain a creep resistance higher than in the conventional case, therefore, the spring hardness should be increased to .phi. 2.50 to 2.70 mm in HBD.
If the hardness of the spring steels exceeds a certain level, however, their fracture toughness lowers, while their notch sensitivity increases. If the spring steels become hard, then their endurance limit is improved in proportion. If the spring hardness becomes harder than .phi. 2.60 mm in HBD, however, the endurance limit is subject to substantial variation. This is supposed to be attributable to lowered fracture toughness. Thus, the spring steels cannot be good for the service if they are only hardened.
Shot peening (hereinafter referred to as SP in some cases) is generally known as means for improving the durability of a spring. The shot peening is a process in which compressive residual stress is produced in the surface of the spring as an object of processing by dashing a number of shots against the spring. Shots harder than the spring must be used in order to produce a sufficient compressive residual stress on the spring surface to determine the durability of the spring.
In the high-hardness spring described above, however, the spring hardness becomes harder than the hardness of normal-hardness shots (about .phi. 2.70 mm in HBD), so that the sufficient compressive residual stress cannot be produced. Accordingly, the shots used must be harder than the spring. In the case of a high-hardness spring with the hardness of .phi. 2.50 to 2.70 mm in HBD, for example, shots with the hardness of .phi. 2.50 mm or more harder in HBD should be used.
The harder the shots, however, the shorter their life is, as shown in FIG. 7. The life of the aforesaid high-hardness shots with the hardness of .phi. 2.50 mm or more in HBD, in particular, is much shorter than that of the conventional normal-hardness shots (.phi. 2.70 mm in HBD), and is not practical at all.
For these reasons, it has been believed that the hardness of the spring steels practically cannot be increased to .phi. 2.50 to 2.70 mm in HBD for lightweight design.